1. Field of the Invention
The present invention relates to a solder powder, a flux, and a solder paste, which have excellent storage stability and ensure excellent properties in and after reflow; and further relates to a soldering method using the solder paste; a soldered circuit board, and a soldered joint product for electronic parts.
This application is based on Japanese Patent Applications No. Hei 10-161854, No. Hei 10-336898, No. Hei 11-26472, No. Hei 11-88935, No. Hei 11-283870, No. Hei 11-343361, No. 2000-86803, and No. 2000-217317, and provisional patent applications Ser. Nos. 60/135,839, 60/135,841, 60/135,842, 60/159,529, and 60/232,432 filed in U.S., the contents of which are incorporated herein by reference.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Solder paste is used for surface mounting of electronic parts in the electronics industry. Solder paste is suitable for automation because of its printing adaptability and tackiness, therefore, the amount thereof used has been increasing in recent years.
In the electronics industry, a solder paste is coated on a printed board by screen printing or, depending on the case, by a dispenser, then electronic parts are mounted thereon and subsequently, the board is subjected to reflow to fix the electronic parts. “Reflow” means a series of operations such that a board on which electronic parts are mounted is preheated and then the solder paste is heated at a temperature higher than its melting point to weld the parts.
On the other hand, miniaturization of electronic articles has been proceeding in recent years and many kinds of fine pitch parts are used, such as 0.3 mm-pitch QFP (quad flat package) type LSI or even CSP (chip size package). To keep up with this tendency, the solder paste must to have capability for extremely fine pitch printing. This requirement is met by reducing the average particle size of the solder metal particles. However, this technique has a problem in that the specific surface area of particles as a whole is increased and reaction between solder metal particles and flux is accelerated, as a result, the storage stability of the solder paste is degraded.
One of the greatest causes of reduction in the storage stability of the solder paste is in the fact that the solder powder preferentially reacts with flux during storage to allow oxidation to proceed and the activator to be consumed, whereby the activity of the flux decreases and due to the reaction product, the viscosity of the solder paste increases. Because of this, the solder paste cannot maintain its suitable printing property and does not melt at the time of reflow.
In order to improve the stability of solder paste, efforts have been heretofore made with an attempt to protect the surface of the solder metal particles and thereby reduce the reactivity of the metal.
For example, a method of covering solder powder with glycerin (see Japanese Examined Patent Application, Second Publication, No. Hei 5-26598) and a method of coating solder powder with a coating agent insoluble or sparingly soluble in the solvent of the solder paste (see Japanese Unexamined Patent Application, First Publication, No. Hei 1-113197) are known. In the latter case, suitable examples of the coating agent include silicone oil, silicone-based high molecular weight compounds, fluorinated silicone oil, fluorosilicone resin and fluorinated hydrocarbon-based polymer compounds.
In addition, a method of coating solder powder with a rosin-based mixture, namely, a resin mainly comprising rosin, incompatible with flux at room temperature but compatible with flux at the soldering temperature is known (see Japanese Unexamined Patent Application, First Publication, No. Hei 3-184698 and Japanese Unexamined Patent Application, First Publication, No. Hei 4-251691).
According to the method disclosed in Japanese Examined Patent Application, Second Publication, No. Hei 5-26598 and Japanese Unexamined Patent Application, First Publication, No. Hei 1-113197, although covering with a relatively large amount of coating agent may be effective in suppressing the oxidation of solder powder, the large amount of coating material is rather disadvantageous for the reflow of solder paste and many solder balls may be generated. Furthermore, there is a strong fear that the coating agent will come off during kneading at the production of solder paste or by treatment such as transportation, printing or the like on use, because only a physical covering is made and the adhesion seems to be very weak.
The coating agent, that is, the resin mainly comprising rosin, disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 3-184698 and Japanese Unexamined Patent Application, First Publication, No. Hei 4-251691 contains by itself many reactive organic acids and cannot successfully protect the metal powder.
Other than these methods, a method of using acidic phosphoric acid ester as an activator for solder paste and an incombustible or flame retardant low volatile diluent having compatibility with the acidic phosphoric acid ester (Japanese Unexamined Patent Application, First Publication, No. Sho 63-90390), adding a phenol-, phosphorus- or sulfur-based antioxidant (Japanese Examined Patent Application, Second Publication, No. Sho 59-22632 and Japanese Unexamined Patent Application, First Publication, No. Hei 3-124092), or using a specific surface active agent (Japanese Unexamined Patent Application, First Publication, No. Hei 2-147194) is proposed. However, when using these methods, the storage stability of the solder paste is not satisfactory.
In recent years, due to environmental problems, a solder paste containing no lead, so-called Pb-free solder paste, is recommended and to cope with this, development is being driven so as to make the solder paste a Pb-free solder paste.
As the most effective Pb-free solder paste, Sn—Zn system solder pastes are drawing attention, however, they are fairly bad in storage stability as compared with usual Pb-based solder paste, for example, oxidation of Zn in the solder powder proceeds or reaction between Zn and flux takes place at room temperature and the viscosity of the paste increases with the passage of time. In particular, it has been found that Zn reacts with halogen generated by the decomposition of an organic halogen compound in the flux at room temperature, so that the storage stability of the solder paste decreases. Furthermore, a halogen compound in the flux and Zn in the solder powder react to generate a small amount of hydrogen gas, and the small amount of hydrogen generated is imprisoned as a gas in the solder fillet, causing a serious problem in reliability.
Furthermore, one of the causes of the deterioration in soldering property, namely that solder metal is oxidized during the preheating of solder paste, during the reflow process or during the melting of the solder metal, as a result, non-melted solder metal may remain on a board or solder balls may be generated.